In this test suite two instances of the SPI peripheral are connected together.
One SPI instance works as a controller, second one is configured as a peripheral.
In each test, both instances get identical configuration (CPOL, CPHA, bitrate, etc.).
Four GPIO loopbacks are required (see overlay for nrf54l15dk for reference):
1. spi22-SPIM_SCK connected with spi21-SPIS_SCK,
2. spi22-SPIM_MISO connected with spi21-SPIS_MISO,
3. spi22-SPIM_MOSI connected with spi21-SPIS_MOSI,
4. spi22-cs-gpios connected with spi21-SPIS_CSN.
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